1. Field of the Invention
The present invention generally relates to a flying capacitor type battery voltage detector mounted on a vehicle, and more particular to a flying capacitor type battery voltage detector in which voltages created by cells of a battery pack is detected one by one in a predetermined order to check the performance of each cell.
2. Description of Related Art
A battery pack creating a high voltage has generally been used in a hybrid vehicle or a fuel cell electric vehicle to reduce a power loss in wires and to lessen weight of cables. The battery pack is composed of hundreds of cells connected with one another in series. In the vehicle using this battery pack, it is necessary that a control unit periodically detects a voltage created by each of the cells of the battery pack to maintain the cells in preferable operating conditions.
When a plurality of voltage detectors separately disposed are used to detect output voltages of a large number of cells of the battery pack, respectively, a size of a circuit of the voltage detectors is enormously enlarged. To reduce the circuit size, a flying capacitor type battery voltage detector has been proposed.
In this flying capacitor type battery voltage detector, a multiplexer selects cells of a battery pack one by one in a predetermined order according to a predetermined time schedule, a flying capacitor holds an output voltage of each selected cell, and a differential amplifier detects the output voltage of the cell sent from the flying capacitor through an output side analog switch.
Because only a single battery voltage detector having one multiplexer, one flying capacitor and one differential amplifier is used to detect output voltages of a large number of cells of a battery pack one by one, a size of the battery voltage detector can be reduced as compared with that of the voltage detectors prepared for the cells, respectively. In addition, because the same battery voltage detector is used for the cells of the battery pack, a degree of an error of an amplification factor and a degree of an offset error caused by the differential amplifier are substantially invariable in the detecting operations of the output voltages of the cells of the battery pack. Therefore, in addition to the simplification of the configuration of the battery voltage detector, the battery voltage detector is advantageous in that the output voltage of each cell detected by the battery voltage detector can be easily corrected.
Published Japanese Patent First Publication No. 2002-148286 discloses a conventional flying capacitor type battery voltage detector. In this battery voltage detector, a battery pack mounted on a vehicle creates a very high voltage of hundreds of volts and is insulated from the ground. Therefore, it is required to dispose a power source system of control circuits independently from the battery pack. For this requirement, a group of photo MOS (metal oxide semiconductor) switches formed in a configuration of discrete units is generally used as a group of analog switches of a multiplexer. That is, the photo MOS switches are separately disposed parallel to one another, and a plurality of pairs of photo MOS switches are used one by one to detect each of output voltages of the cells. In this case, because the battery pack is composed of hundreds of cells serially connected with one another, a large number (for example, hundreds) of photo MOS switches disposed on a substrate (hereinafter, called circuit substrate) are required to detect output voltages of the cells one by one.
The reason that the photo MOS switches are formed in the configuration of discrete units, in other words, disposed parallel to one another is described. Each photo MOS switch requires both a light emitting device (LED) and a silicon photo detector, and voltages, respectively, applied to the photo MOS switches considerably differ from one another because of the high voltage of the battery pack. Therefore, it is difficult to dispose the photo MOS switches in an integrated circuit as non-discrete units. Hereinafter, because the photo MOS switch belongs to a control voltage separation type analog switch, the photo MOS switch is sometimes called an analog switch in this specification.
However, the above-described battery voltage detector has the following problems. Because the multiplexer is composed of a large number of analog switches (or photo MOS switches), the multiplexer is inevitably disposed in a wide area of a circuit substrate. Further, a resistance (hereinafter, called an on-state resistance) of the analog switch set at an on state has a high temperature dependency, and heat received in the multiplexer is mainly transmitted from the circuit substrate. In general, heat of the circuit substrate is propagated according to the thermal conduction in plane directions perpendicular to a thickness direction of the circuit substrate. Therefore, an environment of temperature at each of portions of the circuit substrate differs from those at the other portions, and the analog switches receiving heat from the circuit substrate are set at various temperatures in a wide temperature range. That is, temperatures of the analog switches have a wide dispersion. As a result, because the on-state resistance of the analog switch is highly changed with temperature, the analog switches have various on-state resistances in a wide resistance range. In this case, when electric energy outputted from each cell is charged as an electrostatic capacity to a flying capacitor through a pair of analog switches set at on-state resistances in a short period of time, the output voltage of the cell is reduced in the analog switches by a changeable reduction value dependent from temperatures of the analog switches, and a reduced voltage applied to the flying capacitor is read out as an output voltage of the cell. Therefore, because a large number of analog switches disposed in a wide area have various on-state resistances in a wide temperature range, the reduction values of the output voltages of the cells are widely dispersed, and voltages read out one after another from the flying capacitor as the output voltages of the cells have various values in a wide voltage range. As a result, noises in voltage are superimposed on signals indicating the output voltages of the cells because of a large temperature difference in the circuit area of the multiplexer, and a signal-to-noise (SN) ratio in each signal is undesirably lowered or degraded (first problem).
Further, each of electric potentials of the positive and negative terminals of each cell is transmitted to one of the terminals of the flying capacitor through a cell side wire, an analog switch and a flying capacitor side wire to hold a voltage of the cell in the flying capacitor. Each cell side wire connects one terminal of the corresponding cell and an input terminal of the corresponding analog switch, and each flying capacitor side wire connects an output terminal of the corresponding analog switch and the corresponding terminal of the flying capacitor. After the output voltage of the cell is held in the flying capacitor, the analog switch is turned off. In this case, the flying capacitor side wire is almost or perfectly electrically insulated from the ground and is set at a floating potential condition. Further, an electromagnetic coupling and/or an electrostatic coupling inevitably occur between the flying capacitor side wire and its neighboring wire or terminal so as to superimpose noises caused by electromagnetic induction and/or electrostatic induction on an electric potential signal of the flying capacitor side wire, and/or the flying capacitor side wire inevitably receives electromagnetic waves as noises from the outside so as to superimpose the noises of the external electromagnetic waves on the electric potential signal. As a result of the occurrence of the coupling and/or the reception of the external electromagnetic waves, the electric potential of the flying capacitor side wire set at the floating potential condition is easily changed, and the voltage applied to the flying capacitor is undesirably changed with the electric potential of the flying capacitor side wire. When an output side analog switch directly connected to a differential amplifier is turned on to transmit electric potentials of the terminals of the flying capacitor to the differential amplifier, the differential amplifier detects an incorrect voltage held in the flying capacitor as the output voltage of the cell. Therefore, the SN ratio is further degraded (second problem).
In the same manner, the electric potential of the cell side wire is changed due to the coupling and/or the external electromagnetic waves, thereby the SN ratio being moreover degraded when an incorrect electric potential of the cell side wire is transmitted to the differential amplifier through a turned-on analog switch.
Further more, because the multiplexer is disposed in a wide area of the circuit substrate, the cell side wires and the flying capacitor side wires connected with the multiplexer are inevitably lengthened. Further, because it is required to ensure the electrical insulation of each wire from the other wires, each wire is disposed to be sufficiently spaced from the adjacent wire. As a result, each wire is further lengthened, and a resistance of the wire is increased. Therefore, the flying capacitor sometimes is not sufficiently charged to detect the output voltage of each cell in the differential amplifier. Further, because resistances (hereinafter, called wiring resistances) of the wires have various values in a wide resistance range, the voltages held in the flying capacitor as the output voltages of the cells have various values in a wide voltage range. As a result, the SN ratio deteriorates. Moreover, as well known, noises based on the resistance of each wire are increased with the wiring resistance, so that the SN ratio is further lowered (third problem).
The wiring resistance in the flying capacitor side wire is described in more detail. A portion (or called a potential detecting line) of each cell side wire extending from the corresponding cell to a connector of the circuit substrate is generally made of a cable, so that the wiring resistance of the portion of the cell side wire can be easily reduced. However, the remaining portions (or called potential detecting substrate surface lines) of the cell side wires extending from the connector of the circuit substrate to the analog switches of the multiplexer are disposed on a surface of the circuit substrate in high density, and the flying capacitor side lines extending from the analog switches to the flying capacitor are also disposed on the surface of the circuit substrate in high density. Therefore, the potential detecting substrate surface lines and the flying capacitor side lines are made of thinned wires. As a result, it is difficult to reduce the wiring resistances of the potential detecting substrate surface lines and the flying capacitor side lines, and the SN ratio is further lowered (fourth problem).
In short, in the conventional flying capacitor type battery voltage detector, a large number of photo MOS switches are dispersedly disposed on a surface of a circuit substrate. Therefore, an S/N ratio of a signal indicating an output voltage of each cell of a battery pack is degraded.
Further, Published Japanese Patent First Publication No. H11-272981 discloses a flying capacitor type insulation circuit. In a case where this insulation circuit is used as a flying capacitor type battery voltage detector, it is necessary to dispose a large number of analog switches formed in a configuration of discrete units on a circuit substrate as a multiplexer. Therefore, the same problems described above occur in this circuit.